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We’ve compiled some of those frequent asks into one place, here, so hopefully, if you’ve ever wondered these curious questions, you can get a whole bunch of answers. Welcome to this episode of SciShow Compilation: FAQ!

Hosted by: Hank Green

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What causes migraines? -Rob Margolis, patreon

What’s the story with Schrodinger’s cat? -Rob Margolis, patreon (same dude as above)

Why is it so hard to remember dreams? -anon, tumblr

How can I fall asleep? -gymnast-seal, tumblr

Why do we have baby teeth? -Uwe, patreon

How does hair know when to stop growing? graysweatshirt + anon, tumblr

 Intro (0:00)


Hank: We get asked a lot of questions here at SciShow. Sometimes we get a question that has maybe never been asked before in the history of questions, and sometimes we get questions that are so universally wondered that they get asked over and over and over again. So today we've compiled those frequent asks into one place here, so hopefully if you've ever wondered these curious questions you can get a whole bunch of answers right now.

 What cause migraines? (0:32)

H: Recently Patreon patron Rob Margolis reminded us of two of these questions that come up quite a lot. The first I hope you're not wondering right now, but if you are I hope you recover quickly and can watch this video about what causes migraines.

If you've never had a migraine, you might think it's just a really bad headache. But if you have ever had one or you know someone who gets them, you know that they are much worse than that and much more complicated.

A true migraine is a multi-symptom disorder of the central nervous system that affects the brain. But yes, really bad headaches are a major component of it, probably the single most significant and identifiable component. But usually they last longer than a normal headache, anywhere from four hours to several days and it brings along with it a whole array of other symptoms. Most migraine sufferers experience extreme sensitivity to light and sound and sometimes even smells. They also commonly experience nausea, vomiting, even fainting. What little relief they can find is generally only achieved by being in a still, dark, silent room until the symptoms pass.

And believe it or not it gets worse. Migraines also cause problems both before and after the headache. It's different for everyone but the ordeal can start with symptoms as seemingly minor constipation, weird food cravings, neck stiffness or excessive yawning. As the symptoms worsen, people generally enter a phase called aura, in which they may experience things like visual disturbances like seeing shapes or lights blurred or double vision or even loss of vision, pins and needle sensations in the extremities, weakness and sometimes even slurred speech. Now you may notice that these sound a lot like the symptoms of a stroke and in fact migraines have so many things in common with strokes that doctors sometimes have to do tests to determine which disorder they're dealing with.

After the headache is passed most migraine-sufferers experience a period of weakness and fatigue that can last from a few hours to a few days.

Obviously this isn't the sort of thing that anybody wants to experience, so what causes it? Can it be controlled or at least treated? Doctors think that migraines probably are caused by a sharp drop in your brain's level of serotonin, a neurotransmitter that plays a key role in regulating things like sleep and mood. And once that imbalance strikes it causes a whole cascade of effects. But what actually triggers this imbalance is complicated and uncertain.

We do know that one of the most important factors is genetics. If one or both of your parents has experienced a migraine odds are that you will as well. For reasons that we do not understand women are far more likely to have migraines than men and they're even more likely to experience one during times of hormonal changes, for example puberty, menstruation, ovulation, pregnancy, when using hormonal contraceptives or hormone replacements, and during menopause.

Beyond that everyone's triggers are different. For many people it may depend on their stress, their activity level or sleep schedule, all things in which serotonin plays a role. And still others may be triggered by things as seemingly random as bright lights, loud sounds, unusually strong smells or even weather changes.

The treatment of migraines is further evidence that it's not just a headache. It's true that the headache itself can sometimes be treated with pain relievers although they're often less effective. So in addition to pain relief, migraine sufferers may take medications that try to treat the source of the attacks, like by controlling the constriction of blood vessels in the brain, blood pressure, serotonin levels, and inflammations.

So clearly a migraine is more than just a bad headache, remember that when you hang out with people who get them. If they're in a bad way the biggest favor you can give them is just let them be by themselves in a dark room. You can keep watching SciShow but just do it very quietly.

 What's the story of Schrödinger's Cat? (3:57)

H: Rob's second question is another that comes up a lot but is less painful for humans anyway.

Hello and welcome to "I Don't Think That Means What You Think It Means", where we look at bits of scientific theory that have wiggled their way into popular culture and have taken on a life of their own.

Today, we're talking about Schrödinger's Cat, a famous thought experiment devised by the Austrian physicist Erwin Schrödinger, who helped piece physics back together after Einstein and his crew blew a giant fricking hole in it back in the early 20th century. It's hard to overstate how much of a giant crap circus the 1920s were for physicists. Until then, everything had pretty much just been good old-fashioned Newtonian physics where you could observe objects moving and predict how they react to various forces. But then along comes all this new research into subatomic particles that shows that they don't act predictably at all. In fact, sometimes stuff seems to be two different things at once, like an electron in a beam might act like a particle sometimes and like a wave at other times, and to make things even more... "gngnngh!" the more you try to observe and measure these particles, the less naturally they behave. Safe to say "what" now?

My friends, I shall now introduce you to one of the biggest mind-flogs of quantum mechanics. It's called superposition, the idea that a particle can exist in all of its theoretically possible states at the same time. So Schrödinger came up with this thought experiment to help everybody understand it.

Say you have a cat, and you put the cat into a steel chamber for an hour with a vial of deadly gas, a Geiger counter, a hammer, and a tiny bit of something radioactive. Bear with me here.

Now say that there's a 50/50 chance that one of the radioactive atoms is going to decay and release some radiation within that hour. If one of the atoms decays, the Geiger counter is going to trigger the hammer, shattering the vial of poisonous gas. Really, Schrödinger? This is not the best way to get people behind the idea of fun in the sciences. So, there's a 50% chance that at the end of the hour, that vial has been broken and the cat is dead, and an equally good chance that the vial hasn't been broken, and the cat's just kicking it, wondering what's for supper.

But, what's actually happening in the box?

According to quantum mechanics, any one of those radioactive atoms would be in a superposition of being both decayed and not decayed at the same time, 'cause that's how quantum objects act. So then, that decayed atom will have both killed and not killed the cat, right?

Well, that's the logical conclusion, but a cat, a cat isn't a quantum object. A cat is a big, normal thing that obeys nice, old-fashioned Newtonian laws, so it, just like every other cat in history, is either alive or dead.

Schrödinger's point, at least one of them, is that the two objects are subject to two separate sets of laws that can't be reconciled. In order to know whether the atom has decayed or not, and whether the cat is dead or not, you have to open the box and see, but in quantum mechanics, the state of superposition can't be observed, so when the evil mad scientist finally opens the chamber to observe, the superposition collapses and the outcome is ensured.

Today, Schrödinger's Cat is often talked about as some kind of undead zombie cat, or discussed as if the cat is actually both alive and dead in the box, but Schrödinger's point wasn't to prove that you can make a cat be both alive and dead, but instead to prove that the quantum world doesn't mesh together well with the, you know, like, normal world. And alternatively, the point is that the universe is pretty fricking weird.

There are other interpretations of quantum mechanics that might resolve the paradox, but none of them are particularly easy to test. My favorite, of course, is the many worlds interpretation, that states that at the end of the experiment and at the end of every superposition, new alternate universes are created, in this case, one in which the cat is alive and one in which the cat is dead. And to be clear, I don't like this interpretation because I think it's the most likely one, I like it because I think it's such an excellent plot device for science fiction novels.

 Why is it so hard to remember dreams? (7:29)

H: I have a theory that most people on Tumblr are either struggling to fall asleep or are still in bed and putting off getting out of bed because these next two frequently asked questions are about sleep and dreams and both get asked a lot on our Tumblr.

Dreaming's gotta be one of the weirdest things we do. I mean, I don't wanna diminish all the other strange crap our bodies are capable of, 'cause a lot of it's pretty cracked out. But dreams are a special kind of crazy, like no matter how many dreams you've had in your life, every once in a while you wake up like, "What the hell was that?" But as with everything else, science is helping us understand why we dream, what our brains are up to when they're doing it, and why dreaming may be critically important to the functioning of our waking brain. So try to stay awake or this one, 'cause it's pretty cool.

People have been trying to understand dreams since there were people. But the person we most associate with the science of dreaming is probably Sigmund Freud. In 1899, he wrote The Interpretation Of Dreams, where he suggested that dreams were largely symbolic and allowed us to sort through the repressed wishes that piled up in our unconscious mind.

And most of those wishes, you might have noticed, involve kinda weird sex stuff. Freud was, you know, Freud. It wasn't until the 1950s, when scientists first had tools that allowed them to read the electrical activity of the brain that we began to understand what a dreaming brain was really up to.

Two researchers at the University of Chicago, Eugene Aserinsky and Nathaniel Kleitman, pioneered this research by hooking people up to the new EEG machine and monitoring their brain activity while they slept. What they thought they'd find was that the sleeping brain was a resting brain but what they discovered was exactly the opposite.

They found that brain activity actually fluctuates in a predictable pattern over a period of about 90 minutes. This cycle takes sleepers from an initial period of drifting off, gradually into a really deep sleep with slower brain activity, back into almost wake. The state of sleep where the sleepers were almost awake again was the most interesting. Brain activity in this phase was almost identical to when people were awake. But even more weird, during this stage the subjects became functionally paralyzed. The only parts of their bodies that moved were their eyes, which darted back and forth under their eyelids.

So you know this part already. Aserinsky and Kleitman called this period REM sleep, after the rapid eye movement that characterized it. They also called it paradoxical sleep because the subjects seemed to be awake, according to their brain activity, even though they were basically dead to the world. They probably figured that these names were better than "sexually aroused sleep," which is another common feature of this stage. But another thing that scientists found out was that if REM sleepers were awakened, they reported having really vivid dreams that were often emotionally intense. It wasn't the only stage of sleep in which the subjects dreamed, but it was the time that they reported having the most lifelike dreams.

It turns out that every 90 minutes or so, during the final stage of the sleep cycle, the brain phases into the REM sleep and our brains start creating crazy narratives that might last between 20 and 30 minutes. This is when you have those crazy, really vibrant dreams that you can almost confuse with reality. So, why so busy, sleeping brain? And what's so important about dreaming that you have to paralyze my entire body in order to have realistic dreams?

Well, there are probably several answers, but one of them is that during the periods of dreaming, our brains are making important connections between real-life experience that will help us in our waking lives. These days, researchers are finding that Freud was wrong about dreams in at least one important way: we don't dream much about our hidden desires. We mostly dream about what we did that day.

When we're sleeping, our brains are sorting through what happened while we were awake, deciding which new experiences were important enough to remember and which ones should get tossed, and searching for links between seemingly unrelated events that might help us be a more successful human tomorrow.

And it's actually really important that we do this while we're asleep because our conscious waking brains are generally too controlling to allow this kind of creative problem solving. And this dream-time activity helps our waking brains be better at things that require making connections and thinking outside the box.

Dreams have actually been responsible for some really important inventions and discoveries in history. For instance, Dmitri Mendeleev came up with the system for the structure of the periodic table of elements in a dream after months of grueling conscious thought was getting him nowhere. And research shows that our brains are much better at solving puzzles if they're allowed to take a nap in the middle of doing one.

In a study in 2004, for instance, subjects were asked to search for links between two sets of numbers. The subjects who napped solved the puzzle 60% of the time, whereas only 25% of non-nappers were able to do it. In another study where people were asked to find connections between seemingly unrelated words, those who lapsed into REM sleep between sessions solved 40% more puzzles than those who didn't.

So, dreams are all about making associations and finding patterns that our waking brains have a hard time detecting. But it seems to work slightly differently in non-REM sleep than in REM sleep.

During non-REM sleep, you dream, but the dreams aren't necessarily vivid and they're often about something you've been doing or thinking about a lot. During these stages, people often report dreaming about kind of boring stuff. Like, if you spend a lot of time in the car during the day, that night, you might dream about driving down a long street, stopping at a series of stoplights. This might seem lame, actually, it's pretty lame, but it's kind of useful to the brain in its own way. It's telling itself things that it already knows, like when you're driving a car, you're supposed to stop at stoplights. So in non-REM sleep it's basically reinforcing existing connections.

But in REM sleep, we get to test out that reinforced knowledge in a context that's virtually indistinguishable from real life. It's like our brain running simulations. So, say you've been driving to your grandparents' house in Boca Raton all day. In the non-REM sleep, you spend a good 20 minutes doing boring stuff like practicing stopping at traffic lights. During REM sleep, your brain might have you trying to steer a steamroller through Manhattan from the backseat.

REM dreams can be very lifelike and very stressful, but that's a part of it. A vivid REM dream is an opportunity to safely let us try something difficult. 'Cause our brains aren't here to make friends; our brains are here to win. The evolutionary purpose of dreaming, like the evolutionary purpose of virtually everything else we do, is to make us more successful animals tomorrow than we were yesterday. In REM sleep, the brain is actually trying to experience the future in order to test possibilities.

So, maybe you're making out with your 7th grade algebra teacher on Jay-Z's yacht while wearing a banana suit. What of it? Does it mean you subconsciously wanna make out with your 7th grade algebra teacher? Maybe, but not necessarily. Does the banana suit have something to do with penises? I don't know; I'm not Freud! The thing is, during REM sleep, you could try that experience with no consequences whatsoever.

Another benefit of REM sleep is that it helps us process emotions that our dunderheaded waking brains aren't really equipped to handle. Although the content of our dreams may be wacky, the emotions attached to them are completely real. Remember, your dreaming brain is charged with working on real-life problems, so if you feel really angry at your boyfriend in a dream, chances are you're probably pretty pissed at him or maybe someone else you're close to in real life.

The stories our dreams create are essentially attempts to give our emotions a narrative that can kind of suck the poison out of them and give them a form our brain can deal with better. In fact, there are people who can't experience REM sleep and they often experience other psychiatric disorders.

So, dreams help regulate the traffic between our experiences, our emotions, and our memories, so we can dial down the crazy. And hey, if the outcome makes your rational brain a little bit uncomfortable, well, that's just how the sausage is made, folks.

Now, since I'm on the topic of weird dreams and REM sleep, a lot of you have said that you'd like to know more about what's called lucid dreaming. This is where you become aware of the fact that you're dreaming and can actually direct the narrative of the dream. Since REM sleep is a simulation in the brain, lucid dreaming is basically a simulation that lets a portion of your conscious brain in on the action. Most of us can probably recall at least one lucid dream and about 1 in 10 of us have them regularly. Some lucid dreamers can even communicate with researchers studying them through gestures like eye movements and hand squeezes.

What ultimately separates lucid dreams from regular old REM sleep may lie in the physiology of the brain. During non-REM sleep, the cerebral cortex, that's your gray matter, loses the ability to associate with other parts of the brain. This is probably why those dreams are more boring and less complex, but once a dreamer reaches REM sleep, the cortex becomes active again and begins talking to other areas of the brain except for this one little part of the cortex called the dorsolateral prefrontal cortex; that doesn't reactivate.

This is the region right about at your left temple that's responsible for, among other things, applying memories to other situations, like planning stuff and predicting outcomes. This helps explain why REM sleep dreams are often so weird; your brain literally can't tell what's going to happen next. But during lucid dreaming, the dorsolateral prefrontal cortex actually does wake up, which is probably why we regain a sense of self-awareness and can plot out stories for ourselves.

Some people claim that lucid dreaming can help with reoccurring nightmares or even help cure depression and anxiety. The jury, though, is out on that, but dreaming itself, all kinds of dreaming, is definitely useful and even imperative to the function of the brain. So, if you're still awake, go take a nap or something.

 How can I fall asleep? (16:08)

Michael: The first thing you should know if you're having a hard time getting some shuteye, is that you're wired to sleep regular hours, going to bed the same time each night and waking up at the same time each morning. Having a regular wake-up time seems to correlate pretty highly with the ability to fall asleep consistently.

This is because it keeps you aligned with what's known as your circadian rhythm, your body's natural tendency to stay in sync with the cycles of day and night.

And you know what controls your body's circadian rhythm more than anything else? Light.

A lot of the help you get falling asleep comes from hormones. They lower your heart rate and reduce your blood pressure and basically let you relax.

The key player here is the hormone melatonin, and it's regulated by your exposure to light. In darkness, it flows freely. But when you're exposed to light, whether natural or artificial, the release of melatonin stops.

So you know what that means? No phones or laptops in bed! The light emitted by electronics confuses your body into not knowing that it's time to sleep.

So scientists suggest at least an hour of screen-free time before bed, though I am completely incapable of that myself.

Another obvious enemy of sleep: caffeine. Even though you might think that cup of coffee after dinner might only affect you for an hour or so, studies have shown that caffeine consumption as much as twelve hours before bedtime is linked with insomnia.

And even the way you think about sleep can affect your sleep patterns. Worrying about not getting enough sleep is a common enough cause of insomnia that it has its own name, sleep onset insomnia.

But you know what's really weird? A lot of the time, when we feel like we can't sleep, we actually are sleeping. When scientists rouse patients in the first or second stages of sleep, more than 60% of them say that they weren't sleeping, even though they were.

Now, of course, there's a whole class of medications that will help you sleep, from antihistamines to the pharmaceuticals known as hypnotics, which include Ambien and Lunesta.

But research has shown that while patients can fall asleep faster on hypnotics, the effect is small, adding only about 15 minutes to their sleep times.

Other studies indicate that our minds are significantly more powerful than any medications. In double-blind studies, patients who were simply told that they were taking a sleep drug ended up sleeping far better than patients who were told they weren't.

So, if you want to know how to sleep, the answer is right there in your head.

Now as part of our work answering the world's most asked questions, we asked you, our SciShow viewers, a few questions and one was how many hours per night you sleep.

And it's bad news: Only 10% of you are sleeping more than eight hours per night, and eight and a half is the doctor-recommended amount. And over half of you report having trouble getting to sleep at least once per week.

And now it's time for meaningless correlations! 

The best sleepers for countries where we had enough data to make any sort of judgement were Saudi Arabians, with 76 percent reporting that they experience insomnia infrequently or never. Most of Europe scored better than average, with the Netherlands, Russia, and Spain all sleeping relatively soundly. The English speakers in the US, UK, and Australia all had some of the worst scores.

And, finally, unsurprisingly, our staggeringly unscientific survey reports that people who commonly drink coffee, soda, energy drinks or tea are all more likely to suffer from insomnia.

 Why do we have baby teeth? (19:00)

H: So now you know all about migraines, hypothetical cats, and sleep, on to the serious questions. Why, why do we even have baby teeth?

Missing teeth. Not so cute in the lead singer of The Pogues, but pretty dang cute on a smiling toddler. But why do humans have baby teeth, and why do we lose them? It is weird. It is a weird, weird thing.

Humans, and most mammals, are diphyodonts, meaning we grow two sets of teeth in our lifetimes: a permanent set of adult teeth, and a deciduous set of baby teeth. Deciduous teeth are smaller and fewer in number because a toddler's jaws are tiny, and could never fit in a full set of 32 adult teeth - because that would be super horrible and terrifying, and no one would get anywhere near them. So instead we begin life with 20 smaller teeth which start erupting out of our gums when we're about to turn six months old, and are fully in by the time we're two and a half. Just like our permanent teeth, deciduous teeth grow in pairs, meaning that when two incisors erupt from the lower jaw, you can bet that two incisors are gonna erupt from the upper jaw soon. This allows our mouths to bite down and chew evenly, and helps ensure that our jaws grow and wear down evenly too.

Now, as we get bigger we need new teeth, or there'd just be a bunch of space between all of them, but instead of wedging those new teeth in between the old ones, we lose the old set and grow a whole new one. That's why baby teeth are called deciduous, just like the leaves on deciduous trees, they will shed at a specific stage of development. Four new molars erupt at the back of our mouths when we're around five or six years old. Then, our deciduous incisors, which are right here in the front, fall out and are replaced by permanent incisors. By the time we reach puberty we have an almost-full set of 28 permanent teeth. The last four emerge later in life - these so called "wisdom teeth" are molars in the back of the mouth. They were super helpful some 100 million years ago when our jaws were bigger, but evolution has made our mouths smaller and now these molars crowd out other teeth and can cause pain, which is why a lot of people get 'em yanked out.

Now, being a diphyodont might not be the best way to go. While it sounds nice to have an extra set of something, we only get two sets of teeth in our lives. Polyphyodonts, on the other hand, can grow and regenerate teeth multiple times. These include alligators, fish, even some mammals like elephants, who can regenerate their teeth up to six times to help them enjoy long lives of grinding up plants. But our second set of teeth will just keep wearing and breaking over time, so take care of them while you can, as they're the only ones you're gonna get.

 How does hair know when to stop growing? (21:19)

H: Our final FAQ that we will share with you today is one I'm pretty sure everybody has wondered while thinking about their eyebrow hair or arm hair or any hair.

You're probably quite happy that your armpit hair isn’t dragging on the floor. So it's good that there’s a system to prevent that. But what is that system? 

Us humans grow hair all over our bodies, except on our palms and the soles of our feet. But some of it, like leg hair, stops growing, while the hair on our heads just seems to grow out forever. That’s because every hair on your body goes through the same cycle: growing for a while and then falling out. But each type of hair spends a different amount of time growing, and grows at a different speed. 

Every hair begins the same way, in a phase of the cycle called anagen. During anagen, blood flow starts to ramp up at the base of the follicle, feeding oxygen to specialized stem cells. These cells begin rapidly dividing and producing keratinocytes, which form the root of the hair. As the expanding mass of keratinocytes is pushed toward the surface of the skin, the cells die, releasing a protein called keratin, which holds the strand of hair together. Eventually that strand pops out of your skin. So the visible part of the hair is entirely dead, which is why, thankfully, it does not hurt to get your hair cut, although try explaining that to a three-year-old. 

During anagen, hair can grow up to 1 and a quarter centimeters every month, depending on where it’s located on your body. The second phase is called catagen, and lasts about two weeks. Here, the blood supply is cut off at the bottom of the follicle, which stops the production of new keratinocytes. So for that particular hair the party is over. The follicle then shrinks to about a sixth of its original size and then the existing hair strand is pushed closer to the surface.

The third phase is called telogen, otherwise known as the resting phase, where the follicle remains dormant for one to four months. Finally, the hair is released, or shed, when the follicle dilates, and starts the anagen phase again.

So, how long a hair on your body gets, depends on how long it’s in the anagen phase, and how fast it grows during that time. The hair on your scalp, for example, stays in anagen for two to six years, which is why it can grow so long. Other hair types, like eyebrows and eyelashes and body hair have a short anagen phase, only 30 to 45 days. But they also grow much more slowly, with eyebrows, for instance, growing only 4.2 millimeters every month.

This is my nice eyebrow closeup. Hey.

As for how your hairs know when to grow and when to stop, that’s something scientists are still trying to figure out. It is known that genetics can lead to longer or shorter anagen phases in certain hair types. But the current thinking is that your hairs get their instructions by way of chemical growth signals from stem cells in the skin. And considering how extremely inconvenient it would be for all of the hairs on your body and all mammals' bodies to just continue growing forever, it makes sense that there's a system for making sure they don’t grow too long.

  Outro (23:54)

H: Thank you for watching this frequently asked questions compilation video. Please keep asking questions. You can ask in the comments, on Patreon, Tumblr, Twitter, Facebook, Instagram, by pigeon if you can figure that out, we will have answers for you too. And in the meantime, thank you for getting smarter with us. You can do that more by going to to subscribe.


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